Examples of heat exchange units for clothes dryers, which use the hot, moisture-laden air exhausted from a dryer to preheat the fresh, ambient air drawn into the dryer, are given in U.S. Pat. Nos. 3,859,735; 4,028,817; 4,063,590 and 4,095,349. Such prior art arrangements place an ambient air flow intake conduit in heat exchange relationship with an air flow discharge conduit, so that heat is transferred from the hot exhausted air to preheat the ambient intake air, thus reducing the energy requirements of the dryer.
In the '735 patent, for example, ambient air is preheated by drawing it through upper and lower openings of an elongated shell housing across parallel spaced runs of straight tubing that carry hot exhaust air from the bottom to the top of the housing. In the '530 patent, ambient air is preheated by drawing it through spaced straight channels formed by one side of a corrugated partition which are in heat exchange relationship with intermediate spaced straight channels formed by the other side of the partition and through which hot exhaust air passes. The '590 patent shows a similar arrangement wherein a heat exchange unit is divided by plates into a plurality of sets of alternating first and second straight passageways for directing the hot exhaust air perpendicularly in heat exchange relationship through the ambient inflowing air. And, the '817 patent structure directs the ambient air through parallel tubes along straight paths perpendicular to the flow of exhaust air through the housing of an exhaust air conduit.
In each case, ambient air is flowed through a first conduit in heat exchange relationship with hot exhaust air flowed through a second conduit, one of the conduits being directed along a straight path through the other. Although the prior art structures show the ambient air flows and hot exhaust air flows in longitudinal as well as transverse heat exchange encounters, one conduit does not loop or double back to recross the other to provide multiple transverse crossings of one flow stream relative to the other.
The described structures, also, all show ambient air being drawn from an area adjacent the dryer. This may not be a problem for commercial dryers located in un-conditioned space or for which air is drawn from the unconditioned interior of enclosures into which the backs of the dryers are built. For a home dryer located in an air-conditioned area this may be a problem, however. It does not make sense in summer, for example, for the homeowner to have to pay to cool down the air in the room, then to draw it into the dryer and have to pay to heat it up again. Moreover, when conditioned air is taken out of the room into the dryer, the unconditioned hot air drawn into the room from outside has to be cooled down.
U.S Pat. No. 4,279,082 addresses this problem by providing an air inlet construction for a domestic clothes dryer which has a pair of selectively usable air inlet ports. One port opens outwardly to draw ambient air from the area immediately adjacent the dryer;
the other connects by means of a duct to draw air from a solar heated ridge portion of an unconditioned attic area. The '082 air inlet, however, makes no accommodation for preheating the intake air by means of the exhausted air.